1. Field of the Invention
The present invention discloses a process monitoring system, and more particularly, a process monitoring circuit and process monitoring method to be used for applications such as generation of threshold voltage for non-volatile memory.
2. Description of the Prior Art
For non-volatile memory, each memory cell stores one bit of information. A way to store the one bit is by supporting two states of the memory cell—one state represents a logical “0” and the other state represents a logical “1”. The two states are implemented by having a floating gate above the channel of the memory cell and having two valid states for the amount of charge stored within the floating gate. Typically, one state with zero charge in the floating gate represents the logical “1” and another state with some amount of negative charge in the floating gate represents the logical “0”. Having negative charge in the floating gate causes the threshold voltage of a transistor of the cell to increase. The threshold voltage is the voltage applied to the gate of the transistor in order to cause the transistor to conduct. The stored bit is read by checking the threshold voltage of the memory cell: if the threshold voltage is at a high state then the bit value is “0” and if the threshold voltage is at a low state then the bit value is “1”. If the memory cell stores a value of “1”, the cell typically has a negative threshold voltage. Similarly, if the cell stores a value of “0”, the cell typically has a positive threshold voltage. To correctly identify in which of the two states the memory cell is currently located, a comparison is made against a reference voltage value that is in the middle of the two states, and thus determining if the threshold voltage is below or above the reference value.
Non-volatile memories are not exactly identical in their characteristics and behavior due to variation of fabrication parameters. The variations are referred to as process corner. A group of non-volatile memories having been fabricated at the same time does produce non-volatile memories having the same process corner and thus affecting the threshold voltage of each of the non-volatile memories. Therefore, same programming operation may not be applied to the non-volatile memories. If an incompatible threshold voltage is used for the programming operation, though the cell may work properly in the beginning, after a large number of programming cycles, i.e., 100 times, the non-volatile memory has a tendency to have a spurious programming operation due to the buildup of offset in the threshold voltage.